The invention relates to an augmented magnetic filter or separator. More specifically, the present invention relates to a filter or separator which utilizes a combination of magnetic gradients and electrophoretic migration to remove suspended solids from liquid streams and, in particular, coolant streams for power plants, such as nuclear power plants.
In the operation of power plants, a maintenence problem exists due to the presence of corrosion products which form in the plant cooling system and which are then deposited on the surfaces of the cooling system. The problem is particularly acute in nuclear power plants wherein deposited primary cooling system corrosion products are associated with radiation exposure of operating and maintenance personnel and the high costs of maintenance of such power plants. These primary cooling system corrosion products are solids which are produced primarily by corrosion of the steam generator tubes and to a lesser extent by corrosion of the other stainless steel plant surfaces.
The corrosion product components are circulated with the primary coolant both as a suspended or insoluble solid phase and to some extent as ions in solution. Deposition of the solid phase particles takes place on the core surfaces and while in residence on the core, these solid phase particles become irradiated. Additionally, the soluble corrosion products may precipitate to add to the layer of solid phase particles already on the core. After being irradiated, the solid phase particles on the core become resuspended in the primary coolant solution and are thereafter deposited on the plant surfaces external to the core. There, the irradiated solid phase particles become the most important source of radiation exposure to personnel in the the power plant.
In a pressurized water reactor (PWR) nuclear power plant, both the primary and secondary coolant streams carry a burden of insoluble magnetic corrosion products. These corrosion products also carry a surface charge generally dependent on the pH of the coolant and, at least in the smaller particle sizes, are subject to electrostatic attraction which causes particles to adhere to the surfaces of the system.
The primary cooling system corrosion products are largely nickel ferrite and nickel ferrite with substitutions for part of the nickel of cobalt, manganese and other elements. Substitution of chromium for valence-III iron in the corrosion products has also been indicated. All these ferrites are ferrimagnetic and strongly attracted by a magnetic field gradient. In the secondary cooling system of such a nuclear power plant, as generally is the case for all power plants, the suspended solid corrosion products consist mainly of magnetite (or iron ferrite) which is likewise strongly magnetic. Also present in the secondary coolant system of most power plants is cupric oxide (CuO) which is weakly paramagnetic. All of these insoluble corrosion products have an important adverse effect on power plant availability and their removal from their respective systems is highly desirable.
Since the insoluble corrosion products are primarily magnetic, there has been considerable interest in using a magnetic filter to remove the suspended solids from power plant coolant streams. The applications of magnetic filters to the primary coolant stream of a nuclear power plant were reviewed by Troy et al, "Effects of High-Temperature Filtration on PWR Plant Radiation Fields" in DECONTAMINATION AND DECOMMISSIONING OF NUCLEAR FACILITIES (M. M. Osterhout), Plenum Press, New York, 1980, pages 633-647. Moreover, investigations of the application of magnetic filters to the secondary coolant streams of conventional power plants, but applicable to nuclear plants as well, were reported in articles by Davenport et al, "On-Site Pilot Tests of a SALA-HGMF Magnetic Filter at New England Power Company, Brayton Point Station", Proceedings of International Water Conference (38th), Pittsburgh, PA, 1977, pages 159-173, and Palmer et al, "Magnetic Filtration of Feedwater Corrosion Products", American Power Conference, Chicago, 1978, pages 969-975, and a comprehensive review and evaluation of magnetic filtration for both primary and secondary system purification was sponsored by the Electric Power Research Institute and reported by Troy et al, "Study of Magnetic Filtration Applications to the Primary and Seconary Systems of PWR Plants", EPRI Report, NP-514, May, 1978.
The magnetic filter conventionally used for removing such corrosion products from a power plant coolant stream generally includes a non-magnetic pressure vessel which contains a metal magnetic matrix and which is surrounded by a solenoid and associated magnetic yoke. The metal matrix is constructed so that it presents a large surface area and distorts the otherwise uniform field produced by the solenoid so as to cause steep gradients in the field directed toward the matrix surface. The pressure vessel is connected into the pipe containing the coolant stream so that the stream flows through the matrix and the magnetic particles are attracted to and deposited on the surface of the magnetic matrix.
In order to trap a particle from suspension on the matrix of the magnetic filter, the magnetic force acting on the particle must be greater than the drag force from the stream flowing through the filter. Moreover, it has been shown that for the optimum effect of such a high gradient magnetic filter on small paramagnetic particles, the radius of the matrix element on which the particle is to be collected should be approximately three times the radius of the particle to be captured. Obviously, for practical magnetic filter matrices the approach to optimum design falls off as the radius of the particles to be captured decreases. Thus, for the submicron (near colloidal) fractions of the primary and secondary corrosion products, the magnetic filter tends to become less effective.
It should be noted that in addition to highly magnetic and paramagnetic corrosion products, coolant streams of power plants often contain other suspended particles, such as diamagnetic corrosion products, e.g. cuprous oxide (Cu.sub.2 O) and nonmagnetic particles, e.g. silica, calcium and magnesium oxides, which it is desirable to remove from the coolant stream but which would not be expected to be trapped by a magnetic filter.